Master station in communications system and access control method

ABSTRACT

A communication band is divided into: a beacon period in which every master station transmits a beacon packet in competition with one another; a guaranteed-band period (e.g., a TDMA period or an FDMA period) in which only a specific permitted station is allowed access; and a CSMA period in which every station is allowed access in competition with one another. Plural master stations exchange information of a communication band used in the guaranteed-band period with each other, and based on the respective information, a communication band available to the master station&#39;s own communications system in the guaranteed-band period is calculated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a master station to be used in acommunications system and an access control method. More particularly,the present invention relates to an access control method which is to beused by a plurality of communications system sharing the same channeland which prevents interferences between the communications systems.

2. Description of the Background Art

As a technique for reducing interferences occurring between a pluralityof communications system sharing the same channel, access controlmethods for reducing the influences of interference waves by usingtransmission power control have been proposed, as disclosed in, e.g.,Japanese Laid-Open Patent Publication No. 2002-198834 and JapaneseLaid-Open Patent Publication No. 2003-37556.

Japanese Laid-Open Patent Publication No. 2002-198834 discloses a methodwhere an attenuator is provided in a base station for attenuating signalpower and interference power, while utilizing the transmission power ofa transmitter in a terminal station to ensure that the power level of awireless signal to be input to a receiver attains a reference level.

On the other hand, Japanese Laid-Open Patent Publication No. 2003-37556discloses a method in which a base station which has detectedinterference waves issues interference information to another basestation which is transmitting the interference waves via a LAN network,and the other base station having received the notification lowers itstransmission power based on the interference information.

However, in the case where the above-described communications system isa power line communications system, due to the characteristics of thepower line transmission path, signal attenuation within a device's owncommunications system may far exceed the signal attenuation which causesinterference in another communications system, depending on theconfiguration of the devices which are connected to the network. Inother words, in the case where the technique of Japanese Laid-OpenPatent Publication No. 2002-198834 or Japanese Laid-Open PatentPublication No. 2003-37556 is applied to a power line communicationssystem, if interference between communications systems is preventedthrough power control, some of the devices in the system may becomeunable to perform communications with other devices due to a reducedsignal intensity, depending on the device configuration. In the case ofa wireless communication, too, a similar phenomenon may occur in that anattenuation in the signal intensity due to shielding obstacles may causea drastic drop in the signal intensity despite a small physicaldistance.

With such conventional structures, it is impossible, throughtransmission power control, to suppress interferences for othercommunications systems while maintaining a good communication qualitywithin a device's own communications system. Therefore, there has been aproblem in that the throughput in each communications system is greatlydeteriorated due to interferences between the communications systems,and that it is difficult to control the communication band.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a masterstation and an access control method which, in the presence of aplurality of communications systems sharing the same channel, easilyavoids interferences between the communications systems whileguaranteeing QoS in the communication bands of the communicationssystems, without having to perform transmission power control.

The present invention is directed to a master station for use in asystem environment including a plurality of communications systemssharing a same channel, each communications system including a masterstation and at least one slave station kept under management by themaster station. In order to attain the object mentioned above, themaster station comprises: a communication section, an acquisitionsection, and a determination section.

The communication section performs communications using a communicationband which is divided into: a beacon period in which every masterstation transmits a beacon packet in competition with one another; atime division multiple access (TDMA) period in which only a specificpermitted station is allowed access using a communication band which isallocated through time division; and a carrier sense multiple access(CSMA) period in which every station is allowed access in competitionwith one another, the communications being performed in a cyclecomprising the beacon period, the TDMA period, and the CSMA period. Theacquisition section acquires a state of used communication band in eachof the other communications systems. Based on the states of usedcommunication band acquired by the acquisition section, thedetermination section calculates a communication band available to themaster station's own communications system in the TDMA period, anddetermining whether a communication which is requested by the at leastone slave station is granted or rejected in accordance with thecalculated communication band.

Typically, the acquisition section may acquire the state of usedcommunication or frequency band in each of the other communicationssystems by exchanging information with the other master station usingthe CSMA period, or acquire the state of used communication or frequencyband in each of the other communications systems from a beacon packetreceived from the other master station in the beacon period.

In response to requests from a plurality of slave stations which do notinterfere with one another in performing communications, thedetermination section may grant the requests to perform communicationsduring a same communication slot, whereby it becomes possible toeffectively utilize the communication band.

In particular, in the case where the TDMA period is used as one of thedivided periods, it is preferable that a ratio between the TDMA periodand the CSMA period dynamically varies in accordance with a totalcommunication band required by the specific station.

On the other hand, in the case where the FDMA period is used as one ofthe divided periods, it is preferable that, in response to a requestfrom the at least one slave station, the determination section grantsthe request to perform a communication using an unused frequency band inthe FDMA period. Moreover, in the case where the communication band issubjected to frequency division by a multicarrier communication schemeutilizing a plurality of subcarriers, it would be effective if thenumber of used subcarriers among the plurality of subcarriersdynamically varies in accordance with a total frequency band required bythe specific station.

The processes performed by the component elements of the master stationdescribed above may be regarded as an access control method defining asequence of processing steps. Such a method may be provided in the formof a program for causing a computer to execute the sequence ofprocessing steps. Such a program may be introduced into the computer ina recorded form on a computer readable recording medium. Alternatively,the component elements of the master station described above may beimplemented as an LSI, which is an integrated circuit.

Thus, according to the present invention, the communication band isdivided into a beacon period, a guaranteed-band period (TDMA or FDMA),and a CSMA period, and the allocation in the TDMA or FDMA period isdetermined based on the information of the communication band used byeach communications system. As a result, even in the case where pluralcommunications systems share the same channel, it is possible to easilyavoid interferences between communications systems without having toperform transmission power control, while retaining good QoS in thecommunication bands for the respective communications systems.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary environment of acommunications system to which the present invention is applicable;

FIG. 2 is a block diagram illustrating an exemplary detailed structureof a station;

FIG. 3 is a diagram illustrating periods into which a communication bandis divided;

FIG. 4 is a timing diagram illustrating an access control methodaccording to a first embodiment of the present invention;

FIG. 5 is a flowchart illustrating the access control method accordingto the first embodiment of the present invention;

FIG. 6 is a sequence diagram illustrating a method which utilizes a CSMAperiod;

FIG. 7 is a flowchart of a method which utilizes a CSMA period;

FIG. 8 is a timing diagram illustrating an access control methodaccording to a second embodiment of the present invention; and

FIG. 9 is a diagram illustrating an exemplary network system in whichthe access control method according to the present invention is appliedto high-speed power line transmission.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be specificallydescribed with reference to the figures.

FIG. 1 is a diagram illustrating an exemplary environment of acommunications system to which the present invention is applicable. FIG.1 illustrates an exemplary environment including three communicationssystems 11 to 13 which interfere with one another. The communicationssystem 11 is composed of a master station 111 and a slave station 112;the communications system 12 is composed of a master station 121, aslave station 122, and a slave station 123; and the communicationssystem 13 is composed of a master station 131, a slave station 132, anda slave station 133.

As shown in FIG. 2, each master station or slave station comprises aband management section 21, a control section 22, a data buffer section23, a data transmission/reception section 24, a timer section 25, and anupper interface section 26. The band management section 21 managesvarious information concerning a communication band. The control section22 is in charge of controlling the entire station. The data buffersection 23 temporarily stores various packets. The datatransmission/reception section 24 may transmit packets which are storedin the data buffer section 23, or store received packets to the databuffer section 23. The timer section 25 keeps points of time related tothe data transmission/reception at each station. The upper interfacesection 26 may be, for example, an interface with an upper-level host,or an interface with another medium (e.g., a communications system) asin the fashion of a bridge. The band management section 21, the controlsection 22, and the timer section 25 together compose a determinationsection. The data buffer section 23 and the data transmission/receptionsection 24 together compose an acquisition section. The control section22 and the data transmission/reception section 24 together compose acommunication section.

According to the present invention, the communication band to be used bythe communications systems 11 to 13 is previously divided into a beaconperiod, a guaranteed-band period, and a CSMA period. The beacon periodis a period in which all master stations transmit beacon packets incompetition with one another. The guaranteed-band period is a period inwhich only specific permitted stations are allowed access by using apre-allocated band. In the present invention, applications to a timedivision multiple access (TDMA) technique and a frequency divisionmultiple access (FDMA) technique will be discussed. The CSMA period is aperiod in which all stations are allowed access in competition with oneanother through carrier sensing. As shown in FIG. 3, these periodsrepeat themselves periodically.

By means of the timer section 25, the master station 111, the masterstation 121, and the master station 131 keep the beacon period, theguaranteed-band period, and the CSMA period under management. Typically,system information which defines time allocation for these three periodsis transmitted while being stored in a beacon packet.

Hereinafter, an access control method employing a master station and aslave station having the above structures will be described.

First Embodiment

FIG. 4 is a timing diagram illustrating an access control methodaccording to a first embodiment of the present invention. In the accesscontrol method according to the first embodiment, the communication bandis divided into a beacon period, a TDMA period in which a communicationband (communication slot) to be used is dynamically allocated throughtime division (hereinafter this period will be referred to as a“Dynamic-TDMA period”), and a CSMA (Carrier Sense Multiple Access)period. The ratio between Dynamic-TDMA period and the CSMA period doesnot need to be constant. The ratio may dynamically vary depending on thetotal communication bandwidth required by a particular station which ispermitted to perform communications in the Dynamic-TDMA period.

When a start time of the beacon period is reached, each of the masterstations 111, 121 and 131 transmits its own beacon packet, in apreviously allocated transmission slot. In the example shown in FIG. 4,the master stations 111, 121 and 131 of the communications systems 11,12 and 13 respectively transmit beacon packets 401, 406 and 410 in thisorder. In each beacon packet, a transmission time for the beacon packet,a start time of the beacon period, a start time of the Dynamic-TDMAperiod, a start time of the CSMA period, etc., as governed by the timersection 25, are contained as system information.

The easiest method for determining the timing with which a beacon packetis transmitted by each master station would be allocating fixedtransmission slots in the order by which the communications systems werebooted. The order of booting may be recognized by having the masterstations connected with a wired backbone, or notifying an instance ofbooting to the other communications systems by utilizing the CSMAperiod. It would also be possible to, in the beacon period, performtransmissions between the communications systems in competition with oneanother by utilizing a random access method such as Slotted ALOHA orCSMA. However, in the case where a random access method is used, beaconsmay not be able to be transmitted in some cases, and therefore both themaster station and the slave stations may need to have a systemsynchronization protecting function.

Next, a method for dynamically allocating a communication band(communication time) for each communications system to use in theDynamic-TDMA period will be described with reference to FIG. 5.

Upon receiving a beacon packet from another master station via the datatransmission/reception section 24 (step S501), each of the masterstations 111, 121 and 131 temporarily stores the beacon packet to thedata buffer section 23. Then, from the beacon packet stored in the databuffer section 23, the control section 22 of each of the master stations111, 121 and 131 extracts band information (e.g., the beacon period, theDynamic-TDMA period allocated to the master station's own communicationssystem, the start time of the CSMA period, and the lengths of timeallocated to the respective periods), and stores the extracted bandinformation to the band management section 21 (step S502). From the bandinformation, each of the master stations 111, 121 and 131 can recognizeat which timing the station having received the request can performcommunications in the Dynamic-TDMA period.

Next, each of the master stations 111, 121 and 131 determines whether anew request has been issued in the master station's own communicationssystem (step S503). If anew request has been issued, each of the masterstations 111, 121 and 131 newly calculates a communication band whichcan be used in the master station's own communications system based onthe stored band information, and compares the calculated communicationband against a sum of the communication band which is currently used bythe master station's own communications system and the communicationband for the new request (step S504). If the comparison indicates thatthe aforementioned sum is equal to or less than the calculatedcommunication band, the master station 111 and the master station 131grant the new request (step S505). On the other hand, if theaforementioned sum is greater than the calculated communication band,the master station 111 and the master station 131 check thecommunication(s) which has already been allocated a band to confirmwhether there is any band which does not interfere with thecommunication to occur in response to the new request (step S507).

The underlying rational is that, even in the case where a plurality ofcommunications systems generally interfere with one another, there maystill be local areas which are free of the influences of interferences,and such areas should be utilized effectively. In the example shown inFIG. 1, if the new request is for a communication between the slavestation 132 and the slave station 133, such a communication will notinterfere in any way with the communication between the slave station122 and the slave station 123. Therefore, this new request can begranted while being allocated with the same band as that which isalready allocated to the communication between the slave station 122 andthe slave station 123. Typically, each master station collectsinformation as to which station each slave station in the masterstation's own communications system is receiving a packet from. Thedetermination of the station from which a given packet is transmittedcan be made based on an address or the like.

Thus, if there is any band which will not have mutual interferences withthe communication to occur in response to the new request, the masterstation 111 and the master station 131 grant the new request (stepS505). On the other hand, if there is no band which will not have mutualinterferences with the communication to occur in response to the newrequest, the master station 111 and the master station 131 reject thenew request (step S506).

Now, a method performed at step S504 of calculating a communication bandavailable to a master station's own communications system on the basisof the communication bands being used by the other communicationssystems will be described with respect to a specific example. Forinstance, let us assume that there is a need to obtain a MAC (MediumAccess Control) efficiency of 0.65 and a 20% redundant bandwidth(margin) for retransmission, in the case where the maximum bandwidth inthe Dynamic-TDMA period is 30 Mbps and a total communication bandwidthused by the other communications systems is 6 Mbps. In this case, thetotal communication bandwidth available to all communications systems is15.6 Mbps (=30×0.65×0.8). Therefore, the communication bandwidthavailable to the master station's own communications system iscalculated to be 9.6 Mbps (=15.6−6.0). Thus, in this example, if a newrequest only entails a communication band of 9.6 Mbps or less, thatrequest will be granted.

As a method for acquiring information of the communication bandwidthbeing used by the other communications systems, a technique utilizingthe CSMA period as follows may be used, instead of the aforementionedtechnique utilizing the beacon period. This technique will be describedwith reference to FIGS. 6 and 7.

For example, if it becomes necessary for the slave station 122 to securea certain QoS level, the slave station 122 transmits a QoS requestpacket 611 for the master station 121 of the same communications system(step S701). Having received the packet 611 via the datatransmission/reception section 24, the master station 121 temporarilystores the information contained in the packet 611 (i.e., parameterssuch as the address of the requesting slave station 122 and therequested bandwidth) to the data buffer section 23. Then, the controlsection 22 of the master station 121 transmits status request packets612 and 614, respectively, to the master station 111 and the masterstation 131, which are already recognized by the master station 121 tobe present in the neighborhood based on the beacon packets stored in thedata buffer section 23 (step S702). Specifically, the control section 22of the master station 121 generates the packets 612 and 614 in the databuffer section 23, and transmits the packets 612 and 614 to the masterstations 111 and 131, respectively, via the data transmission/receptionsection 24.

Having received the packets 612 and 614, the respective datatransmission/reception sections 24 of the master station 111 and themaster station 131 store the packets to the respective data buffersections 23. Then, the respective control sections 22 of the masterstation 111 and the master station 131 transmit status reply packets 613and 615, containing information of the used communication bands asstored in the respective band management sections 21, to the masterstation 121. Specifically, the respective control sections 22 of themaster station 111 and the master station 131 generate the packets 613and 615 in the respective data buffer sections 23, and transmit thepackets 613 and 615 to the master station 121, via the respective datatransmission/reception sections 24.

Having received the packets 613 and 615 from the master station 111 andthe master station 131 (step S703), the control section 22 of the masterstation 121 determines whether the request from the slave station 122can be granted or not, based on the information of used communicationbands contained in the packets 613 and 615 as well as the aforementionedmaximum bandwidth in the Dynamic-TDMA period and the margin (step S704).Moreover, the control section 22 of the master station 121 checks thecommunication(s) which has already been allocated a band to confirmwhether there is any band which does not interfere with thecommunication to occur in response to the new request (step S707). Basedon the results of the above determination and checking, the controlsection 22 of the master station 121 generates a QoS reply packet 616containing grant of the request or rejection of the request in the databuffer section 23, and sends the QoS reply packet 616 to the slavestation 122 via the data transmission/reception section 24 (steps S705,S706). At the same time, the control section 22 of the master station121 updates the band information which is managed in the band managementsection 21.

If the request is granted, the master station 121 transmits to the slavestation 122 band information indicating at which point in theDynamic-TDMA period transmission is granted, the band information beingcontained in a beacon packet. The slave station 122 detects in thecontrol section 22 that the band information in the received beaconpacket has been updated, and transmits data with the designatedtransmission timing. Thus, data transmission can be performed withoutcompeting with the other stations.

The grant of the request or rejection of the request to the slavestation 122 is also notified to the other master stations 111 and 131 bytransmitting QoS change notice packets 617 and 618 thereto,respectively. If the received packets 617 and 618 necessitate that thestart time of the Dynamic-TDMA period used by each master station's owncommunications system be changed, the other master stations 111 and 131update the band information which is managed in the respective bandmanagement sections 21.

Then, having received the packet 616 indicating grant of the request,the control section 22 of the slave station 122 in the Dynamic-TDMAperiod transmits data packets using a predetermined communication band.A recipient station which has successfully received the data packetsreturns an acknowledgement packet.

In the first embodiment, in order to perform packet transmission(packets 402, 403, etc.), each communications system further applies astation-by-station time division to the band which is allocated in theDynamic-TDMA period. Instead of time division, any other multiple accessscheme, such as frequency division or code division, may be employed.The first embodiment illustrates an example where the Dynamic-TDMAperiod is subjected to a communications-system-by-communications-systemtime division; alternatively, a station-by-station time division may beapplied (where “station” refers to any station which has transmitted aband request). Furthermore, the first embodiment illustrates an examplewhere a desired rate is notified as the band information; alternatively,a desired transmission time which is considered necessary to satisfy adesired rate while taking the media state into account may be notified.

Second Embodiment

FIG. 8 is a timing diagram illustrating an access control methodaccording to a second embodiment of the present invention. In the accesscontrol method according to the second embodiment, the communicationband is divided into a beacon period, an FDMA period in which acommunication band to be used is dynamically allocated through frequencydivision (hereinafter this period will be referred to as a “Dynamic-FDMAperiod”), and a CSMA period.

When a start time of the beacon period is reached, each of the masterstations 111, 121 and 131 transmits its own beacon packet, in apreviously allocated transmission slot. In the example shown in FIG. 8,the master stations 111, 121 and 131 of the communications systems 11,12 and 13 respectively transmit beacon packets 801, 806 and 810 in thisorder. In each beacon packet, a transmission time for the beacon packet,a start time of the beacon period, a start time of the Dynamic-FDMAperiod, a start time of the CSMA period, etc., as governed by the timersection 25, are contained as system information. The method fordetermining the transmission timing of a beacon packet for each masterstation is as described above in the first embodiment.

Next, a method for dynamically allocating a communication band(frequency bandwidth) for each communications system to use in theDynamic-FDMA period will be described. The following description assumesthat, within the frequency bandwidth available to each communicationssystem, a multicarrier communication method in which communications areperformed by using a plurality of narrow-band subcarriers is carriedout. It is also assumed that an OFDM technique using 400 subcarriers, towhich subcarrier Nos. 1 to 400 are assigned, is employed for instance.

Upon receiving a beacon packet from another master station via the datatransmission/reception section 24, each of the master stations 111, 121and 131 temporarily stores the beacon packet to the data buffer section23. Then, from the beacon packet stored in the data buffer section 23,the control section 22 of each of the master stations 111, 121 and 131extracts band information (e.g., the beacon period, the Dynamic-FDMAperiod allocated to the master station's own communications system, thestart time of the CSMA period, and the lengths of time allocated to therespective periods), and stores the extracted band information to theband management section 21. From the band information, each of themaster stations 111, 121 and 131 can recognize, in the Dynamic-FDMAperiod, at which timing the station having received the request canperform communications.

Next, each of the master stations 111, 121 and 131 determines whether anew request has been issued in the master station's own communicationssystem. If a new request has been issued, each of the master stations111, 121 and 131 confirms the frequency bandwidth (subcarrier(s))available to the master station's own communications system based on thestored band information, and determines whether there exists anysubcarrier which can be used for the new request. If the result of thedetermination indicates that such a subcarrier(s) exists, the masterstation 111 and the master station 131 grant the new request. On theother hand, if no such subcarriers exist, the master station 111 and themaster station 131 check the subcarriers which have already beenallocated to confirm whether any subcarrier exists that does notinterfere with the communication to occur in response to the newrequest. The determination as to the influences of interferences can bemade in the manner described above.

Now, a method of calculating a communication band available to a masterstation's own communications system on the basis of the communicationbands being used by the other communications systems will be describedwith respect to a specific example. For instance, let us assume that themaximum bandwidth in the Dynamic-FDMA period is 40 Mbps; the totalcommunication band being used by the other communications systems is 6Mbps; and subcarrier Nos. 300 to 400 are being used. Let us furtherassume that there is a need to obtain a 40% margin for the sake ofretransmission or for absorbing differences in efficiency, which dependson the frequency characteristics of each subcarrier. Therefore, in thiscase, the total communication band which is available to allcommunications systems is 24 Mbps (=40×0.6). Therefore, thecommunication band available to the master station's own communicationssystem is calculated to be 18 Mbps (=24−6.0), corresponding tosubcarrier Nos. 1 to 299. Thus, in this example, if a new request onlyentails a communication band of 18 Mbps or less, that request will begranted.

As a method for acquiring information of the communication bandwidthbeing used by the other communications systems, a technique utilizingthe CSMA period as follows may be used, instead of the aforementionedtechnique utilizing the beacon period. This process is similar to thatshown in FIG. 7.

For example, if it becomes necessary for the slave station 122 to securea certain QoS level, the slave station 122 transmits a QoS requestpacket for the master station 121 of the same communications system.Having received the packet via the data transmission/reception section24, the master station 121 temporarily stores the information containedin the packet (i.e., parameters such as the address of the requestingslave station 122 and the requested bandwidth) to the data buffersection 23. Then, the control section 22 of the master station 121transmits status request packets to the master station 111 and themaster station 131, which are already recognized by the master station121 to be present in the neighborhood based on the beacon packets storedin the data buffer section 23.

24.

Having received the packets, the respective data transmission/receptionsections 24 of the master station 111 and the master station 131 storethe packets to the respective data buffer sections 23. Then, therespective control sections 22 of the master station 111 and the masterstation 131 transmit status reply packets, containing information of theused communication bands as stored in the respective band managementsections 21, to the master station 121. Each status reply packetcontains minimum required band information, used subcarrier Nos., andthe like, which are requested in the respective communications system towhich the master station 111 or the master station 131 belongs.

Having received the packets from the master station 111 and the masterstation 131, the control section 22 of the master station 121 determineswhether the request from the slave station 122 can be granted or not,based on the information of used communication bands contained in thepackets as well as the aforementioned maximum bandwidth in theDynamic-FDMA period and the margin. Moreover, the control section 22 ofthe master station 121 checks the communication(s) which has alreadybeen allocated a band to confirm whether there is any band which doesnot interfere with the communication to occur in response to the newrequest. Based on the results of the above determination and checking,the control section 22 of the master station 121 generates a QoS replypacket containing grant of the request or rejection of the request inthe data buffer section 23, and sends the QoS reply packet to the slavestation 122 via the data transmission/reception section 24. At the sametime, the control section 22 of the master station 121 updates the bandinformation which is managed in the band management section 21.

If the request is granted, the master station 121 transmits to the slavestation 122 band information indicating at which subcarrier No. and atwhich point in the Dynamic-FDMA period transmission is granted, the bandinformation being contained in a beacon packet. The slave station 122detects in the control section 22 that the band information in thereceived beacon packet has been updated, and transmits data by using thedesignated subcarriers, with the designated transmission timing. Thus,data transmission can be performed without competing with the otherstations.

The grant of the request or rejection of the request to the slavestation 122 is also notified to the other master stations 111 and 131 bytransmitting QoS change notice packets thereto. If the received packetsnecessitate a change in the used subcarrier number information in theDynamic-FDMA period used by each master station's own communicationssystem, the other master stations 111 and 131 update the bandinformation which is managed in the respective band management sections21.

Then, having received the packet indicating grant of the request, thecontrol section 22 of the slave station 122 in the Dynamic-FDMA periodtransmits data packets using a predetermined communication band. Arecipient station which has successfully received the data packetsreturns an acknowledgement packet.

In the second embodiment, in order to perform packet transmission(packets 802, 803, etc.), each communications system further applies astation-by-station time division to the band which is allocated in theDynamic-FDMA period. Instead of time division, any other multiple accessscheme, such as frequency division or code division, may be employed. Byemploying frequency division, in particular, the only information whichis necessary for the station-by-station allocation of communication bandis the used subcarrier number information, so that the overheadassociated with control packets can be reduced. In a communicationssystem where band allocation is not performed in the Dynamic-FDMAperiod, it is also possible to perform communications through randomaccess, by utilizing the unused subcarriers.

As described above, in accordance with the access control method of thepresent invention, the communication band is divided into a beaconperiod, a guaranteed-band period, and a CSMA period, and the allocationin the guaranteed-band period is determined based on the information ofthe communication band used by each communications system. As a result,even in the case where plural communications systems share the samechannel, it is possible to easily avoid interferences betweencommunications systems without having to perform transmission powercontrol, while retaining good QoS in the communication bands for therespective communications systems.

The technique utilizing the Dynamic-TDMA period described in the firstembodiment and the technique utilizing the Dynamic-FDMA period describedin the second embodiment may be used in combination. By performing bandallocation in terms of both time and frequency, it becomes possible toconstruct a system which is flexible with respect to the temporalcharacteristics and frequency characteristics within each communicationssystem.

Note that the above-described embodiments can be realized by causing aCPU to execute program data, which is able to cause a CPU to execute theabove-described procedure, stored in a recording medium (a ROM, a RAM,or a hard disk, etc.). In this case, the program may be executed afterit is stored in a storing device via a recording medium, or may bedirectly executed from the recording medium. Here, the recording mediumincludes a ROM, a RAM, a semiconductor memory such as a flash memory, amagnetic disk memory such as a flexible disk and a hard disk, an opticaldisk memory such as a CD-ROM, a DVD, and a BD, a memory card, or thelike. The “recording medium” as mentioned herein is a notion including acommunication medium such as a telephone line and a carrier line.

Note that all or part of the functional blocks composing the masterstation according to the present invention may typically be realized asan LSI, which is an integrated circuit (which may be referred to as anIC, a system LSI, a super LSI, or an ultra LSI, etc., depending on thedegree of integration). Each functional block may be separatelyconstructed in a chip form, or all or some of the functional blocks maybe constructed in a chip form.

Also, the method of integration is not limited to LSI, and may berealized by a dedicated circuit or a general purpose processor. Also, anFPGA (Field Programmable Gate Array), which is an LSI that can beprogrammed after manufacture, or a reconfigurable processor enablingconnections and settings of the circuit cells in the LSI to bereconfigured may be used.

Further, in the case where another integration technology replacing LSIbecomes available due to improvement of a semiconductor technology ordue to the emergence of another technology derived therefrom,integration of the functional blocks may be performed using such a newintegration technology. For example, biotechnology may be applied to theabove-described integration.

Hereinafter, an example in which the above-described embodiments areapplied to an actual network system will be described. FIG. 9 is adiagram illustrating an exemplary network system in which the accesscontrol method according to the present invention is applied tohigh-speed power line transmission. In FIG. 9, IEEE1394 interfaces, USBinterfaces, or the like of multimedia devices such as a personalcomputer, a DVD recorder, a digital television, a home server system areconnected to a power line via a module which is provided with thefunctions according to the present invention. As such, a communicationnetwork system is configured to transmit digital data such as multimediadata at high speed via a power line. As a result, it is possible to usea power line, which has already been installed in a home and an office,etc., as a network line without the need for installation of a networkcable unlike in a conventional cable LAN. Thus, the present inventioncan be easily installed at low cost, thereby substantially improvinguser-friendliness.

The above embodiment is an example in which, via an adapter forconverting a signal interface of an existing multimedia device to apower line communication interface, the existing device is applied topower line communications. However, in future, the functions accordingto the present invention may be built into multimedia devices, wherebyit becomes possible to perform data transmission between the devices viaa power cord of the multimedia devices. In this case, as shown in FIG.9, wiring for connecting an adapter, an IEEE1394 cable, a USB cable canbe eliminated, whereby wiring can be simplified. Also, since connectionto the Internet via a router or connection to a wireless or cable LANvia a hub is also possible, it is possible to extend a LAN system usingthe high-speed power line transmission system according to the presentinvention. Also, since communication data is transmitted over a powerline, it is possible to prevent leakage and interception of data, unlikein a wireless LAN. Thus, the power line transmission method isadvantageous in terms of data protection from a security standpoint. Itwill be understood that data transmitted over a power line may beprotected by an IPSec, which is an extended IP protocol, encryption ofcontents, other DRM schemes, and the like.

Thus, it is possible to perform a high-quality power line transmissionof AV contents by realizing copyright protection by encryption ofcontents, and by implementing a QoS function encompassing the effects ofthe present invention (improved throughput, and band allocation whichflexibly supports increases in retransmission and traffic fluctuations).

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

1. A master station for use in a communications system including at least one slave station in a system environment in which the communications system and a plurality of other communications systems share a same channel, each of the other communications systems comprising a respective other master station and at least one slave station, said master station comprising: a communication section for performing communications using a first communication band which is divided into: a beacon period in which said master station and all other master stations transmit a beacon packet in competition with one another; a time division multiple access (TDMA) period in which only specific permitted stations are allowed access using a communication band which is allocated through time division; and a carrier sense multiple access (CSMA) period in which every station is allowed access in competition with one another, the communications being performed in a cycle comprising the beacon period, the TDMA period, and the CSMA period; an acquisition section for acquiring a state of used communication band in each of the other communications systems; and a determination section for, based on the states of used communication band in the other communications systems acquired by the acquisition section, calculating a second communication band available in the communications system to which said master station belongs, in the TDMA period, and determining whether a communication which is requested by the at least one slave station of the communications system to which said master station belongs is granted or rejected in accordance with the calculated second communication band; wherein a ratio between the TDMA period and the CSMA period dynamically varies in accordance with a total communication band required by a station.
 2. The master station according to claim 1, wherein the acquisition section acquires the state of used communication band in each of the other communications systems by exchanging information with the other master stations using the CSMA period.
 3. The master station according to claim 1, wherein the acquisition section acquires the state of used communication band in each of the other communications systems from a beacon packet received from another master station in the beacon period.
 4. The master station according to claim 1, wherein the determination section receives requests from a plurality of slave stations of the communications system to which said master station belongs, determines whether the plurality of slave stations interfere with one another in performing communications, and grants the requests to perform communications during a same communication slot when the determining section determines that the plurality of slave stations making the requests do not interfere with one another in performing communications.
 5. A master station for use in a communications system including at least one slave station in a system environment in which the communications system and a plurality of other communications systems share a same channel, each of the other communications systems comprising a respective other master station and at least one slave station, said master station comprising: a communication section for performing communications using a communication band which is divided into: a beacon period in which said master station and all other master stations transmit a beacon packet in competition with one another; a frequency division multiple access (FDMA) period in which only specific permitted stations are allowed access using a communication band which is allocated through frequency division; and a carrier sense multiple access (CSMA) period in which every station is allowed access in competition with one another, the communications being performed in a cycle comprising the beacon period, the FDMA period, and the CSMA period; an acquisition section for acquiring a state of used frequency band in each of the other communications systems; and a determination section for, based on the states of used frequency band in the other communications systems acquired by the acquisition section, calculating a frequency band available in the communications system to which said master station belongs, in the FDMA period, and determining whether a communication which is requested by the at least one slave station of the communications system to which said master station belongs is granted or rejected in accordance with the calculated frequency band; wherein: the communication band is subjected to frequency division by a multicarrier communication scheme utilizing a plurality of subcarriers, and a number of used subcarriers among the plurality of subcarriers dynamically varies in accordance with a total frequency band required by the specific station.
 6. The master station according to claim 5, wherein the acquisition section acquires the state of used frequency band in each of the other communications systems by exchanging information with the other master stations using the CSMA period.
 7. The master station according to claim 5, wherein the acquisition section acquires the state of used frequency band in each of the other communications systems from a beacon packet received from another master station in the beacon period.
 8. The master station according to claim 5, wherein in response to a request from the at least one slave station of the communications system to which said master station belongs, the determination section grants the request to perform a communication using an unused frequency band in the FDMA period.
 9. The master station according to claim 5, wherein the determination section receives requests from a plurality of slave stations of the communications system to which said master station belongs, determines whether the plurality of slave stations interfere with one another in performing communications, and grants the requests to perform communications using a same frequency band when the determining section determines that the plurality of slave stations making the requests do not interfere with one another in performing communications.
 10. An access control method to be executed by a master station for use in a communications system including at least one slave station in a system environment in which the communications system and a plurality of other communications systems share a same channel, each of the other communications systems comprising a respective other master station and at least one slave station, wherein communications are performed using a first communication band which is divided into: a beacon period in which said master station and all other master stations transmit a beacon packet in competition with one another; a time division multiple access (TDMA) period in which only specific permitted stations are allowed access using a communication band which is allocated through time division; and a carrier sense multiple access (CSMA) period in which every station is allowed access in competition with one another, the communications being performed in a cycle comprising the beacon period, the TDMA period, and the CSMA period, the method comprising: acquiring a state of used communication band in each of the other communications systems; based on the acquired states of used communication band in the other communications systems, calculating a second communication band available in the communications system to which said master station belongs, in the TDMA period; and determining whether a communication which is requested by the at least one slave station of the communications system to which said master station belongs is granted or rejected in accordance with the calculated second communication band; wherein a ratio between the TDMA period and the CSMA period dynamically varies in accordance with a total communication band required by a station.
 11. An integrated circuit internalized in a master station for use in a communications system including at least one slave station in a system environment in which the communications system and a plurality of other communications systems share a same channel, each of the other communications systems comprising a respective other master station and at least one slave station, wherein the integrated circuit comprises circuitry functioning as: a communication section for performing communications using a first communication band which is divided into: a beacon period in which said master station and all other master stations transmit a beacon packet in competition with one another; a time division multiple access (TDMA) period in which only specific permitted stations are allowed access using a communication band which is allocated through time division; and a carrier sense multiple access (CSMA) period in which every station is allowed access in competition with one another, the communications being performed in a cycle comprising the beacon period, the TDMA period, and the CSMA period; an acquisition section for acquiring a state of used communication band in each of the other communications systems; and a determination section for, based on the states of used communication band in the other communications systems acquired by the acquisition section, calculating a second communication band available in the communications system to which said master station belongs, in the TDMA period, and determining whether a communication which is requested by the at least one slave station of the communications system to which said master station belongs is granted or rejected in accordance with the calculated second communication band; wherein a ratio between the TDMA period and the CSMA period dynamically varies in accordance with a total communication band required by a station.
 12. A communication managing device operable to manage a communication performed by a first communication device and connect to a second communication managing device, which manages a communication by a second communication device, via a transmission line, said communication managing device comprising: a communication section for performing communications using a communication band which is divided into: a first communication band which is a beacon period, a second communication band which is a time division multiple access (TDMA) period, and a third communication band which is a carrier sense multiple access (CSMA) period; an acquisition section for acquiring, via the transmission line, information concerning a communication band used by the second communication managing device; and a control section for controlling at least one of the first communication band, the second communication band, and the third communication band based on the information acquired by the acquisition section; wherein a ratio between the TDMA period and the CSMA period dynamically varies in accordance with a total communication band required by a station.
 13. The communication managing device according to claim 12, wherein the control section controls the second communication band based on the information acquired by the acquisition section.
 14. The communication managing device according to claim 12, wherein the acquisition section acquires information concerning the second communication band which is used by the second communication managing device.
 15. The communication managing device according to claim 13, wherein the acquisition section acquires information concerning the second communication band which is used by the second communication managing device. 